Abnormal repetitive behaviors (stereotypies, self-injury, compulsions) are foremost among the varieties of aberrant behavior exhibited by individuals with mental retardation. Despite this, little is known about their pathobiology and efficacious somatic treatments are largely unconfirmed by controlled trials. For example, few controlled studies have examined the efficacy of pharmacological treatment of abnormal repetitive behavior in individuals with mental retardation. There is evidence, however, to hypothesize the efficacy of both 5-HT uptake inhibitors and atypical antipsychotics. Currently, there is little information to guide the clinician in deciding which drug class may be more effective for which categories of abnormal repetitive behaviors and for which individuals. Moreover, little work has attempted to identify variables that may predict differential treatment response. Thus, the focus of this proposal is to assess the relative efficacy of a selective serotonin reuptake inhibitor (SSRI) and an atypical antipsychotic across multiple categories of abnormal repetitive behaviors. In addition, we will build upon our previous work to establish sets of measures that (a) will provide novel information about repetitive behavior disorders, (b) will predict drug response, and (c) that will serve as outcome measures of drug efficacy. The specific aims will include: 1) identifying the temporal structure and environmental dependence of repetitive and non-repetitive behavior disorders; 2) identifying physiological (motor, autonomic) variables that discriminate subjects with and without discrete categories of abnormal repetitive behaviors; 3) evaluating the differential efficacy of an atypical antipsychotic and a selective serotonin reuptake inhibitor (SSRI) in treating abnormal repetitive behaviors in individuals with mental retardation; 4) evaluating the utility of temporal, environmental and physiological variables as both predictors of drug response and outcomes of pharmacotherapy.